With the observation angle of TFT-LCD increasing, the image contrast has been reduced continuously, and the image definition is decreased. This is due to the birefringence of liquid crystal molecules in the liquid crystal layer changing along with the observation angle. Compensating with a compensation film of a wide viewing angle can reduce light leakage of the dark state image effectively, and increase the image contrast significantly in a certain range of viewing angles. The compensation film (or called the delay film) has the general compensation principle of correcting the phase difference generated by the liquid crystal at different viewing angles, so that the birefringent property of the liquid crystal molecules is symmetrically compensated. Different compensation films are used for different liquid crystal display modes. The compensation films used in a large-sized liquid crystal TV are mostly targeted in the VA display mode, including N-TAC of the Konica company in the early stage, and Zeonor of the OPOTES company, the F-TAC series of the Fujitsu company, X-plate of the Nitto Denko, etc. developed continuously afterwards.
FIG. 1 is an exploded drawing of the display device in the existing technology. As shown in FIG. 1, this display device includes a VA (Vertical Alignment) liquid crystal unit 300 and an optical compensation structure, the latter further including a front side polarizer 100 and a rear side polarizer 200 arranged at both sides of the VA liquid crystal unit 300. Wherein the front side polarizer 100 is arranged at the side in the light-exiting direction of the VA liquid crystal unit 300 (here the light-exiting direction is defined to be the direction of the backlight source 400 oriented to the viewer 500), including a front side polarization unit 110 as well as a second front side TAC film 120 arranged between the front side polarization unit 110 and the VA liquid crystal unit 300. The rear side polarizer 200 includes a rear side polarization unit 220 as well as a rear side biaxial film 210 arranged between the rear side polarization unit 220 and the VA liquid crystal unit 300, the rear side biaxial compensation film 210 having the function of both a delay film and a protection film.
The view angle of the liquid crystal display device (e.g. a liquid crystal TV) is usually called a viewing angle range, which includes two indexes, i.e. a horizontal view angle and a vertical view angle. The horizontal view angle of the liquid crystal display device refers to an angle range within which the display image can still be normally seen at a position perpendicular to the normal to the left or right at a certain angle with the vertical normal of the liquid crystal unit as the reference. Similarly, if the horizontal normal of the liquid crystal unit is taken as the reference, the up-and-down view angle is called the vertical view angle. For convenient description in this application, the horizontal normal 300a of the VA liquid crystal unit is used as the reference for calibration of the direction.
In the existing optical compensation structure, the absorption axis 110a of the front side polarization unit 110 is parallel to the horizontal normal 300a, and the slow axis 120a of the second front side TAC film 120 is perpendicular to the horizontal normal 300a; the absorption axis 220a of the rear side polarization unit 220 is perpendicular to the horizontal normal 300a, and the slow axis 210a of the rear side biaxial compensation film 210 is perpendicular to the horizontal normal 300a. 
The above optical compensation structure includes one layer of biaxial compensation film (the rear side biaxial compensation film 210), and is therefore also usually called a single-layered biaxial compensation film compensation structure. FIG. 2 is a dark-state light leakage profile of the single-layered biaxial compensation film compensation structure. It shows the full viewing angle dark-state light leakage distribution of the single-layered biaxial compensation film compensation structure, indicating that there is serious light leakage at phi=20°-40°, phi=140°-160°, phi=200°-220° and phi=310°-330°, i.e. there is serious dark-state light leakage at a viewing angle close to the horizontal position. However, the position of a viewer relative to the liquid crystal display device determines that the viewer is easier to see at a viewing angle close to the horizontal position. Therefore, the contrast and definition at the horizontal viewing angle have the biggest influence on the viewing effect.
In order to resolve the above problem, the solution commonly used at present is substituting the double-layered biaxial compensation film compensation structure for the above single-layered biaxial compensation film compensation structure. Still as shown in FIG. 1, in the double-layered biaxial compensation film compensation structure, the biaxial compensation film is substituted for the original second front side TAC film 120, making both the front side polarizer 100 and the rear side polarizer 200 as a polarizer with the biaxial delay film.
FIG. 3 is a dark-state light leakage profile of the double-layered biaxial compensation film compensation structure. It shows the full viewing angle dark-state light leakage distribution of the double-layered biaxial compensation film compensation structure, indicating that there is serious light leakage at phi=30°-60°, phi=120°-150°, phi=210°-240° and phi=300°-330°, which are between the horizontal and vertical viewing angles. Generally, the contrast and definition at the horizontal viewing angle have the biggest influence on the viewing effect, while it is difficult to view at a big viewing angle (close to the vertical viewing angle), which thus has smaller influence on the viewer. It can thus be seen that using a double-layered biaxial compensation film compensation structure can improve the viewing effect.
Besides, FIGS. 4 and 5 show respectively the full viewing angle contrast profiles of the single-layered biaxial compensation film compensation structure and the double-layered biaxial compensation film compensation structure. It can be seen from FIGS. 4 and 5 that, the contrast of the double-layered biaxial compensation film compensation structure in FIG. 5 close to the horizontal viewing angle is significantly improved compared with that of the single-layered biaxial compensation film compensation structure in FIG. 4 close to the horizontal viewing angle, with the display effect further improved.
However, although the double-layered biaxial compensation film compensation structure improves the display effect, it will increase the cost of the display device, which makes the display device thus produced in an inferior position in the market competition.